Pencil Water Bag Experiment

If you’ve ever seen someone shove a handful of sharpened pencils straight through a plastic bag full of water without a single drop spilling out, you probably wondered if it was some kind of magic trick. Good news: it’s not magic at all—it’s science, and surprisingly simple science at that.

So… why does this work?

Pin

The Science Behind the Trick

1. Polymers Are Flexible and Self-Sealing

Ziploc bags are made from polyethylene, which is a type of polymer.
Polymers are long, flexible chains of molecules. Imagine a bowl of cooked spaghetti: the strands slide and stretch around one another rather than snapping.

When you push a pencil through the bag:

• The polymer chains stretch and rearrange around the pencil.
• They form a tight seal against its surface.
• This prevents gaps from forming where water could escape.

It’s a bit like how a water balloon can stretch around your fingers—but polyethylene is tougher and less likely to tear.

Pin

---

2. Pressure Works In Your Favor

Inside the bag, the water pushes outward, but the pencil acts as a plug.
Because the plastic forms a tight seal around it, the internal pressure actually helps hold the seal in place rather than forcing water out.

---

3. The Pencil Stays Put

The trick only works as long as you leave the pencils in the bag.
Remove them, and the stretched plastic can’t reseal perfectly—so yes, you’ll get a leak.

---

Tips for Success

• Use smooth pencils. A rough or cracked surface can tear the plastic.
• Insert the pencil in one confident, steady push. Slow works, but hesitation can cause tearing.
• Choose freezer-grade bags. They’re thicker and more durable.
• Avoid old or sun-damaged bags. They tear more easily.

---

Why This Makes a Great Demonstration

This simple experiment beautifully illustrates:

• Polymer flexibility
• Material properties
• Pressure dynamics
• Everyday “science magic”

It’s perfect for classrooms, science fairs, or just impressing kids in the kitchen with Elf!

Watch it live…